Generally speaking, the portions and structural components of a human ear with which the present invention is most closely concerned, though well known to those skilled in the art, are illustrated diagrammatically (without being drawn precisely to scale) in FIGS. 1A and 1B of the hereto appended drawings. In the human ear 30 (see FIG. 1A) of a normal hearing person, sound impinges on the eardrum 31 and is transmitted into the cochlea 32 via a system of bones 33 called the ossicles, which act as levers to provide amplification and acoustic impedance matching, to a piston or membrane 34 called the oval window. The cochlea 32 is a spirally wound tube, resembling a snail shell, which is about 35 mm long when unrolled and is divided along most of its whole length (see FIG. 1B) by a partition 35 called the basilar membrane. The lower chamber 36 of the cochlea is called the scala tympani, and the upper chamber 37 is called the scala vestibuli. The cochlea is filled with a fluid with a viscosity of about twice that of water. The scala tympani 36 is provided with another piston or membrane 38 called the round window (see FIG. 1A), which serves to take up the displacement of the fluid when the oval window 34 is moved.
When the oval window is acoustically driven via the ossicles 33, the basilar membrane 35 is correspondingly displaced and vibrated by the movement of fluid in the cochlea. The displacement of the basilar membrane stimulates the hair cells 39 which are situated in a special structure 39a on the basilar membrane. Movements of these hairs produce electrical discharges in fibers of the auditory nerve through the intermediary of the cells 40 of the spiral ganglion 41 which are located in the modiolus or modiolar wall 42 at the radially inner wall 43 of the cochlea.
Hearing loss, which may be due to many different causes, is generally of two types, conductive and sensorineural. Of these, conductive hearing loss occurs where the normal mechanical pathways for sound to reach the hair cells in the cochlea are impeded, for example, by damage to the ossicles. Conductive hearing loss may often be helped by use of conventional hearing aids, which amplify sound so that acoustic information does reach the cochlea and the hair cells. Some types of conductive hearing loss are also amenable to alleviation by surgical procedures.
In many people who are profoundly deaf, however, the reason for their deafness is sensorineural hearing loss. This type of hearing loss is due to the absence or the destruction of the hair cells in the cochlea which are needed to transduce acoustic signals into auditory nerve impulses. These people are unable to derive any benefit from conventional hearing aid systems, no matter how loud the acoustic stimulus is made, because their mechanisms for transducing sound energy into auditory nerve impulses have been damaged. Thus, in the absence of properly functioning hair cells, there is no way auditory nerve impulses can be generated directly from sounds. To overcome this problem, there have been developed numerous cochlear implant systems which seek to bypass the hair cells in the cochlea (the hair cells are located in the vicinity of the radially outer wall 44 of the cochlea) by presenting electrical stimulation to the auditory nerve fibers directly, leading to the perception of sound in the brain and an at least partial restoration of hearing function. The general common denominator in these systems has been the implantation, into the cochlea, of electrodes which are responsive to suitable external sources of electrical stimuli and which are intended to transmit those stimuli to the ganglion cells and thereby to the auditory nerve fibers.
In the known cochlear implant systems, the carrier 45 for the stimulating electrodes 46, as shown in FIGS. 2-7, usually is a slightly tapered straight or minimally curved rod of a cylindrical or part-cylindrical cross-section and is made of a resiliently flexible biocompatible synthetic plastic material such as a silicone polymer available commercially under the name "Silastic". The electrode carrier is surgically placed into the scala tympani, in close proximity to the basilar membrane, and currents passed to the electrodes 46 via respective conductors or leads 47 embedded in the electrode carrier result in neural stimulation in proximate groups of ganglion cells. For this purpose, the electrodes or contacts, which are secured to the electrode carrier at spaced locations along its length, should preferably be located as close as possible to the modiolus, i.e., near the radially inner wall 43 of the cochlea, where the spiral ganglion cells 40 to be stimulated are located.
Here, however, a problem has been encountered. The electrode carriers, as already mentioned, are generally manufactured in a straight tapered rod-like form from a resilient polymer or like elastomeric material. By virtue of their being straight or only very slightly curved, the electrode carriers can be smoothly and easily inserted into the scala tympani 36 of the cochlea 32 through the opening in the round window 38 or through a small hole drilled into the basal part of the cochlea. When such an electrode carrier 45 (see FIG. 10) is so inserted into the cochlea 32, it flexibly curves into the spiral form of the scala tympani, but because the electrode carrier is resilient and has a "memory" tending to return it to its straight form, it ends up lying against, and closely following the curvature of, the radially outer wall 44 of the cochlea. As a result, the electrodes 46 on the carrier end up being located in the vicinity of the damaged and non-functional hair cells 39 on the basilar membrane 35 (see FIG. 11) but at a substantial distance (relatively speaking) from the inner wall 43 of the cochlea and hence also relatively far from the ganglion cells 40 in the modiolus, thereby limiting the achievable stimulation of the ganglion cells. In such a case, it is necessary to use stimulation currents which are somewhat higher than is usually deemed desirable, but that in turn leads to an undue overspread of the current and a reduction in the resolution of the stimulus.
In an attempt to overcome these drawbacks, which also have not been overcome by proposals to make the electrode carrier either in a more or less greatly curved form such as a spiral shape (see FIGS. 8 and 9) approximating the curvature of the cochlea and utilizing the memory of the carrier material to return the carrier to that shape after the carrier has been straightened for the purposes of the insertion operation, it was proposed in the aforesaid prior application to make the conventional straight rod-like electrode carrier in the form of two layers. Of these layers, the one which incorporates the electrical contacts or electrodes and their leads (herein designated the inner layer) was described as being made of a biocompatible silicone polymer, e.g., Silastic, which does not expand or swell when exposed to the water in the patient's body fluids, while the other layer (herein designated the outer layer) was described as being suitably adhered to the inner layer at the side of the latter directed away from the contact faces of the electrodes and as being made of a biocompatible silicone polymer (also Silastic) formulated through the addition of finely ground NaCl or polyacrylic acid or the like so as to have the property of expanding (swelling) under the action of the water in the patient's body fluids once the carrier has been inserted into the cochlea. By means of such an arrangement, it was suggested, the outer layer of the electrode carrier would, due to the liquid-generated expansion of the outer layer, be ultimately shifted away from its initial position at the radially outer wall of the cochlea to a second position within the scala tympani where the inner layer has its concavely curved surface, at which the contact faces of the electrodes are located, disposed in engagement with the radially inner wall of the cochlea and thereby in close proximity to the modiolus and the ganglion cells. As a solution to the indicated problem, however, this approach was not perfect because, even at best, achieving a post-implantation precise control of the expansion of the outer layer and of the resultant curvature of the inner layer of the electrode carrier is extremely difficult. Moreover, if an unduly great deviation from the desired curvature were to occur, this might necessitate corrective action by means of additional invasive surgery.